Japanese Unexamined Patent Application Publication No. 2006-60909 discloses a power transmission system using magnetic-field coupling and detecting whether a foreign object is on a coupling coil on a power transmission side. The power transmission system described in Japanese Unexamined Patent Application Publication No. 2006-60909 includes a power transmission device, detects a voltage applied to the coupling coil in the power transmission device, and detects the presence or absence of a foreign object on the basis of fluctuations in the detected voltage.